This invention relates in general to fans. More specifically it relates to an axial flow fan designed for low noise operation in a turbulent air flow such as the flow exiting an automobile radiator.
One problem inherent in the design of highly skewed fan blades of a shape addressed in this invention is that often a significant load from centrifugal forces is applied to the blades. Another problem is recirculation of air from the downstream, high pressure region of the blades' working surfaces to the upstream, low pressure side of the fan. One route for this recirculation is around the outer ends of the blades. Any significant level of recirculation may cause the flow to separate. This condition is often evidenced by a higher degree of noise. Some of these design problems, especially the higher noise levels, are enhanced when the fan operates in a turbulent air flow.
In automotive applications, a fan is usually situated behind a radiator where it draws air from the radiator. While efficiency of operation is important in any automobile since an enhanced airflow is more effective in cooling the radiator, the noise level generated by the fan is also an important design consideration. This importance of noise is particularly applicable for vehicles where the fan may continue to operate after the engine is turned off, or where the other noise sources have been intentionally quieted to a degree that the fan is the dominant noise source.
Conventional automotive fans, like most room air fans, have a set of blades secured at a root end to a hub that is driven by a rotating shaft. The blades are usually "straight" or unskewed, that is, the blade chord is generally uniformly distributed about a radial centerline of the blade and the centerline is straight. The fan blades are angled or pitched to propel the air in an axial direction when they rotate. Often the blades are stamped from sheet metal and they may have some degree of camber in addition to their pitch.
While most automotive fans currently in use fall in the foregoing category, at least one fan employs an outer ring that surrounds the blades and is secured to the outer ends of the blades. This construction provides enhanced mechanical support for the blades. U.S. Pat. No. 1,441,852 to Heintz shows a much earlier automotive fan using an outer rim for the same purpose. The rim is a thin ring of structural material secured to the blades by narrow connecting strips. The blades are straight, but their pitch increases from the hub to the rim and they are cambered. Outer support rings are occasionally used in room fans, as for example in U.S. Pat. No. 818,804 to Winch. In Winch, however, the outer ring is used to support blades formed of a flexible material suspended between the hub and the ring.
Marine propellers have also been deisgned using an outer ring that supports the propeller blades. U.S. Pat. Nos. 5,364; 506,572; 1,518,501; and 2,270,615 are illustrative. Besides a support function, in the '364 patent the "outer casing" limits a tangential flow of water off the working surface of the blades and in the '501 patent the outer "shroud" acts as a nozzle.
In both the marine art, and to a lesser extent in the fan art, it is also known to skew the blade, that is, to curve a blade with respect to its root centerline. The '501 patent is illustrative of a moderately forwardly skewed blade. Also in the '501 design, the blade pitch decreases steadily from the hub to the outer "shroud". In air fans skewed blades appear to have been used principally in old room fans. Applicant is aware of no axial flow air fan designed for low noise operation that utilizes both forwardly skewed blades and an outer support member secured to the ends of the blade.
Applicant notes, however, that marine propeller designs involve different design considerations than air fans. A propeller design that performs well in water will not necessarily be of value as an air fan design. One reason for this situation is that almost all propellers are designed for use where the vessel is moving forward in the water. This movement results in the propeller blades being "lightly loaded", i.e. imparting only a small increase to the fluid velocity. There is usually no corresponding movement of air fans. Design assumptions and principles must also reflect the differences in the working fluid--air versus water. For example, cavitation is an important problem in propeller design which is not present in fan design.
It is therefore a principal aspect of this invention to provide a fan that is designed for low noise operation in a turbulent airflow, as compared to conventional fans of similar size and function.
A further object of this invention is to provide a fan system that has an increased efficiency in that it can provide an increased air flow against the same system pressure drop relative to a conventional fan using the same drive power or an equivalent airflow at a reduced power consumption.
Yet another object is to provide a fan system with the foregoing advantages that controls air recirculation from regions of high air pressure to regions of low air pressure.
A still further object of the invention is to provide a fan system with the foregoing advantages that is simple to construct, mechanically strong, has a comparatively low weight and has a competitive cost of manufacture.